X-ray imaging technology has been employed in a wide range of applications from medical imaging to detection of unauthorized objects or materials in baggage, cargo or other containers generally opaque to the human eye. X-ray imaging typically includes passing radiation (i.e., X-rays) through an object to be imaged. X-rays from a source passing through the object interact with the internal structures of the object and are altered according to characteristics of material the X-rays encounter. By measuring changes in the X-ray radiation that exits the item, information related to characteristics of the material in the item, such as density, atomic structure and/or atomic number, etc., may be obtained.
To measure atomic number, X-ray radiation exiting the object is measured at two or more energy levels. Because materials of different atomic numbers respond differently to X-rays of different energy levels, measuring interaction at multiple X-ray energy levels provides an indication of the atomic number of the material with which the X-ray radiation has interacted. In some X-ray inspection systems used for security screening of baggage or other items, dual energy measurements are used in combination with density measurements to classify objects within an item under inspection. Such systems may use automated detection algorithms to analyze X-ray images that detect objects and classify them as threat or non-threat objects based on size, shape, density and material composition. These systems are called “dual energy systems” because useful distinctions between materials can generally be made using any two energy levels. Though, some dual energy systems make measurements at more than two energy levels.
The energy level of X-rays is determined by characteristics of the components used to generate the X-ray radiation. Some X-ray inspection systems have sources that use electron beams as part of their X-ray generation subsystems. In these systems, an e-beam is directed to impinge on the surface of a target that is responsive to the e-beam. The target may be formed from or plated with, tungsten, molybdenum, gold, metal, or other material that emits X-rays in response to an electron beam impinging on its surface. The target material is one factor that can impact the energy of emitted X-rays. A second factor is a voltage used to accelerate electrons toward the target. An electron beam may be generated, from an electron source called a cathode and a voltage may be applied between the cathode and target to accelerate electrons toward the target.
Some inspection systems employ multiple X-ray generation components, each configured to emit radiation at a different energy level. Though, other inspection systems may employ a switching power supply to change the voltage level within one X-ray generation subsystem to control the subsystem to emit X-rays of different energy levels at different times.
An alternative approach for making multi-energy X-ray measurements is to use different types of detectors. Some detectors are preferentially sensitive to radiation of a specific energy level. The output of such detectors can be taken as an indication of radiation at those energy levels. By illuminating an item under inspection with X-ray radiation over a broad spectrum, the output of detectors sensitive to different energy radiation may be used to form dual energy measurements.
In addition to classifying systems based on whether they form single energy or dual energy images, inspection systems may be classified based on the type of images they form. Multiple types of X-ray inspection systems are known. Two types are projection imaging systems and volumetric imaging systems. In a projection imaging system, an X-ray generating component is positioned on one side of an item under inspection and detectors are positioned on an opposite side. Radiation passes through the item under inspection predominately in a single direction. As a result, an image formed with a projection imaging system is a two-dimensional representation of the item, with objects inside the item appearing as if they were projected into a plane perpendicular to the direction of the X-rays.
In contrast, in a volumetric imaging system, radiation passes through the item under inspection from multiple directions. Measurements of the radiation exiting the item under inspection are collected and, through computer processing, a three-dimensional representation of objects within the item is computed. One class of volumetric imaging system is called a computed tomography (CT) system.
Conventional CT systems establish a circular relationship between an X-ray generating component and X-ray detectors. One approach for forming the circular relationship is to mount both the X-ray generating component and detectors on a rotating gantry that moves relative to the item under inspection. An alternative approach is to control an X-ray generating component to alter the location from which it emits X-ray radiation. Such control can be achieved in an e-beam system by steering the e-beam to strike different locations on the target at different times.
An e-beam may be steered magnetically by bending the beam using one or more magnetic coils, herein referred to as steering coils. In general, the e-beam propagates in a vacuum chamber until the e-beam impinges on the target. Various methods (e.g., bending an electron beam using one or more magnets) of providing an e-beam along a desired path over a surface of the target are well known in the art.